Why is standard plastic encapsulation bad for RF?

Standard ICs are protected by pouring liquid epoxy resin over the die, which hardens into a black plastic brick (Overmolded Plastic). Epoxy has a high dielectric constant and terrible loss tangent. When a 20 GHz RF signal attempts to travel through the tiny gold bond wires, the surrounding plastic acts like a terrible capacitor, absorbing the RF energy and converting it to heat. At microwave frequencies, standard plastic packaging will completely destroy the signal before it even leaves the chip.

How does an air cavity solve this?

Air has a perfect dielectric constant of 1.0 and virtually zero dielectric loss. By placing the transistor die inside a ceramic or Liquid Crystal Polymer (LCP) 'tub' and sealing a lid over the top, the bond wires remain surrounded entirely by air. Because there is no heavy dielectric material pressing against the wires, parasitic capacitance is minimized, and the 20 GHz signal can exit the package cleanly without being absorbed.

Are air cavity packages better for heat dissipation?

It depends entirely on the base material. The air itself is a terrible thermal conductor, so heat cannot escape out the top of the chip. However, air cavity packages are typically built with solid Copper-Tungsten (CuW) or pure Copper flanges on the bottom. The semiconductor die is soldered directly to this massive metal base. So while the top of the chip cannot radiate heat, the bottom provides vastly superior thermal conductivity compared to standard plastic packages.

Passive Components

Air Cavity Package

A semiconductor company designs a cutting-edge 100-Watt GaN transistor for 5G base stations. During internal probing on the bare silicon die, the transistor performs flawlessly at 6 GHz with 70% efficiency. However, when they package the die in standard overmolded plastic to sell it, the efficiency plummets to 40%. The dense epoxy resin encasing the microscopic gold bond wires adds massive parasitic capacitance, detuning the carefully designed internal matching networks and absorbing the RF energy. To restore performance, they switch to an Air Cavity Package. They mount the die onto a solid copper flange, surround it with a ceramic wall, and glue a ceramic lid over the top, leaving the internal die and bond wires suspended in pure air. Because air has a dielectric constant of 1.0, the parasitic capacitance vanishes. The transistor returns to 70% efficiency and is ready for the market.

Category: Passive Components

Dielectric Constant (internal): 1.0 (Air/Nitrogen)

Primary Advantage: Eliminates epoxy-induced parasitic capacitance at high frequencies

RF Packaging Comparison

Package Type	Internal Environment	Parasitic Capacitance	Typical Max Frequency	Cost
Overmolded Plastic (e.g., standard QFN)	Solid Epoxy Resin (ε_r ~ 4.0)	High	~ 6 GHz	Very Low
Air Cavity LCP (Plastic)	Hollow Air/Nitrogen (ε_r = 1.0)	Low	~ 15 GHz	Moderate
Air Cavity Ceramic	Hollow Air/Nitrogen (ε_r = 1.0)	Very Low	> 40 GHz	Very High

Bond Wire Parasitics:
The capacitance of a bond wire to the ground plane is directly proportional to the dielectric constant (ε_r) of the material surrounding it.
C ∝ ε_r
If you replace air (ε_r = 1.0) with standard epoxy mold compound (ε_r = 4.0), the parasitic capacitance of every single bond wire instantly quadruples. At microwave frequencies, this massive capacitance acts as a dead short to ground, destroying the signal.

The "Q" Factor Drop:
High-power transistors use internal L-C matching networks inside the package. The Quality Factor (Q) of those networks dictates efficiency. Because epoxy resin has a terrible loss tangent (tan δ), it burns the RF energy oscillating in the LC network as heat, destroying the Q factor. Air has zero loss tangent.

Common Questions

Frequently Asked Questions

Why are air cavity packages so expensive?

Manufacturing. A standard plastic package is made by placing a thousand chips in a mold and shooting liquid plastic over all of them simultaneously—it costs pennies. An air cavity package requires assembling a physical "box." The die must be placed inside a pre-fired ceramic ring, bonded, and then a lid must be epoxied or soldered onto the top with extreme precision to ensure a hermetic seal. This piece-by-piece physical assembly drives the cost up exponentially.

Is it really just air inside?

Usually, no. Standard atmospheric air contains moisture. If the chip gets cold, the moisture would condense into water droplets on the die, causing electrical shorts and corrosion. High-end air cavity packages are assembled in vacuum chambers and back-filled with ultra-dry, inert Nitrogen gas before the lid is sealed. This guarantees absolute zero moisture inside the cavity.

What is an LCP Air Cavity?

Liquid Crystal Polymer (LCP) is an advanced plastic that has excellent RF properties. Ceramic air cavity packages are the gold standard for high frequencies, but they are incredibly expensive. LCP air cavity packages are a modern compromise. The "box" and the lid are molded out of LCP plastic instead of ceramic. It provides the same hollow-air benefits but is much cheaper to mass-produce, making it popular for high-volume 5G infrastructure.

Related Terms

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Passive Components

Package Parasitic Extractor

Input your bond wire dimensions, frequency, and select the encapsulation material (Air, Epoxy, or LCP). Instantly calculate the parasitic capacitance multiplier and the expected drop in matching network Q-factor caused by dielectric loading.

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